[Erasmus]

Actually, the first calculators and similar devices were PMOS. I don't know the details, but I think the process was easier. Also, PMOS could run at higher voltages, allowing them to drive vacuum fluorescent and neon-discharge based displays directly, whereas NMOS or CMOS could not (easily).

[Lexinom]

Interesting. Yes, I remember those displays. Also the popular ADC0800 analog-to-digital converter was PMOS - probably for same reason of higher voltages.

You're the first person, including my EE professors, who has offered a plausible explanation; thanks!